What If? Lab
What if the Sahara turned green?
The Sahara is a big, dry desert. Long ago it was green, with grass and lakes! What if it turned green again? Let's imagine it with Olly.
Thousands of years ago, the Sahara Desert was a land of grass, wetlands and lakes, where cave paintings show hippos in watering holes. A slow wobble of Earth on its axis brought stronger monsoon rains. Find out why the Sahara was green, why it dried, and what might change if it turned green again.
During the African Humid Period, roughly 11,000 to 5,000 years ago, the Sahara was likely far wetter, with vegetation, wetlands and perhaps large lakes (NASA Earth Observatory). Climate modeling links such humid periods to orbital precession and a stronger West African Monsoon. This thought experiment separates that evidence from ideas about a green Sahara today.
The big question
The Sahara is a huge desert in northern Africa.
Long ago, it was green, with grass and lakes. What if it turned green again?
The Sahara is a desert in northern Africa that covers more than 3.5 million square miles. Today much of it is a band of sand, rock and scrub.
But the Sahara has not always looked like this. Scientists have found signs that it was once green, with grass, wetlands and lakes. In this thought experiment you can follow three paths: what the green Sahara of long ago was like, which animals lived by its water, and why it changed. Then you can imagine what might happen if it turned green again.
The Sahara spans more than 3.5 million square miles of northern Africa (MIT News). Yet geologic and archaeological evidence, NASA Earth Observatory notes, indicates that vegetation, wetlands and perhaps large lakes once covered areas that are now seas of sand.
This thought experiment starts from the African Humid Period, asks what drove it and why it ended, and then reasons about what might change if the Sahara greened again. The past is evidence. The future part is idea, because even the direction of future Saharan rainfall is uncertain: some parts may become wetter and others drier.
Explore the idea
Make a prediction
Long ago, what helped make the Sahara green?
What brought the extra rain that made the Sahara green long ago?
What strengthened the rainfall behind the African Humid Period?
- A stronger summer monsoon
- Rivers from melting glaciers
- People watering the desert
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Established fact
The Sahara was green long ago, with grass and lakes.
Roughly 11,000 to 5,000 years ago, the Sahara was likely far wetter and greener, with plants, wetlands and perhaps large lakes (NASA).
The African Humid Period brought vegetation, wetlands and perhaps large lakes to the Sahara, and ended within one to two centuries (NASA Earth Observatory; MIT News).
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What the model shows
A computer model shows the wobble of Earth bringing rain.
A climate model links green Sahara periods to Earth's 21,000-year wobble and a stronger monsoon (University of Bristol).
Armstrong et al. (2023) simulate precession-paced humid periods over 800,000 years, suppressed during glacials.
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An idea scientists discuss
A green Sahara today is pretend. We imagine what might happen.
What might happen if the Sahara turned green today is an idea built from what scientists know about its past.
Every consequence of a renewed green Sahara here is reasoned from past evidence and might differ greatly in reality.
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Established fact
Long ago, the Sahara had plants, wetlands and maybe big lakes.
Roughly 11,000 to 5,000 years ago, the Sahara was likely far wetter and greener, with plants, wetlands and perhaps large lakes.
Geologic and archaeological evidence indicates vegetation, wetlands and perhaps large lakes during the African Humid Period, roughly 11,000 to 5,000 years ago (NASA Earth Observatory).
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Established fact
The green time ended, and the Sahara dried in one or two hundred years.
The African Humid Period likely lasted from 11,000 to 5,000 years ago and ended suddenly: the region dried within one to two centuries.
The humid period is thought to have ended within one to two centuries; ocean sediment records from McGee and colleagues show it began and ended very abruptly, nearly simultaneously across North Africa (MIT News).
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Established fact
The green Sahara made much less dust than today.
During the green period, the Sahara gave off about five times less dust than it does today.
Dust emissions during the African Humid Period were about one-fifth of today's levels (McGee and colleagues, Earth and Planetary Science Letters).
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An idea scientists discuss
If the Sahara turned green again, less dust might blow away.
If the Sahara turned green again, plants might cover the ground, and much less dust might blow into the sky.
A renewed humid phase might cut dust emissions as it did before; how far, and how fast, cannot be estimated here.
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Established fact
Old cave paintings in the Sahara show hippos in watering holes.
Ancient cave paintings in the Sahara show hippos in watering holes and roving herds of elephants and giraffes.
Rock art in the region depicts hippos in watering holes and herds of elephants and giraffes (MIT News, 2013).
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Established fact
In 2024, rain filled part of a dry lake in the Sahara.
After heavy rain in September 2024, Sebkha el Melah, a usually dry lake in Algeria, was roughly one-third full by October 16.
By October 16, 2024, Sebkha el Melah held water over 191 km² to a depth of 2.2 m, roughly one-third full (Moshe Armon, Hebrew University of Jerusalem).
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An idea scientists discuss
With more rain, lakes might stay full, and animals might drink there.
With rain every year, desert lakes might stay full, and animals that need water might find homes there again.
Sustained rainfall might keep basins like Sebkha el Melah filled for long periods; which animals might return depends on far more than water and cannot be judged here.
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Established fact
Earth wobbles slowly. The wobble repeats about every 21,000 years.
Precession is how Earth slowly wobbles on its axis. It repeats about every 21,000 years and changes how much energy Earth gets in each season.
Precession modulates seasonality over an approximate 21,000-year cycle, controlling the strength of the African Monsoon (University of Bristol, 2023).
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What the model shows
A computer model made the Sahara green again and again, every 21,000 years.
A climate model showed North African humid periods every 21,000 years over the past 800,000 years, but not during the ice ages.
The simulation, using a recently developed climate model, reproduced humid periods paced by precession over 800,000 years, suppressed when large ice sheets cooled the atmosphere (Armstrong et al., 2023).
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An idea scientists discuss
Many thousands of years from now, the wobble might bring more rain again.
Because the wobble keeps repeating, the Sahara might one day become greener again, but no one can say when.
Future orbital forcing might again favor a stronger monsoon, but greenhouse warming is layered on top, and NASA notes it is unclear whether or how much the Sahara might green.
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Established fact
Dust from the Sahara falls on the Amazon rainforest and helps its plants.
On average, about 27.7 million tons of Saharan dust fall over the Amazon basin each year, carrying about 22,000 tons of phosphorus, a plant nutrient.
CALIPSO data (2007–2013): 182 Mt/yr of dust leaves the Sahara at 15°W and 27.7 Mt/yr settles over the Amazon; its ~22,000 t of phosphorus roughly balances losses to rain and flooding (Yu and colleagues, NASA).
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Established fact
Scientists are not sure if the Sahara will get more rain or less.
Climate projections show some parts of the Sahara getting more rain and others less, and the uncertainty is larger than the change.
IPCC projections suggest some parts of the Sahara may get more rainfall and others less; Armon notes the uncertainties are larger than the projected changes (NASA Earth Observatory).
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An idea scientists discuss
A green Sahara might send less dust to the rainforest.
A greener Sahara might send less dust across the ocean, so the Amazon might receive less phosphorus from Africa.
Reduced Saharan dust might lower phosphorus delivery to Amazon soils; whether other sources might make up the difference is beyond this experiment.
In our world
Today the Sahara is mostly sand and rock.
Wind lifts its dust high into the sky. The dust can fly across the sea!
In 2024, a big storm filled part of a dry lake.
Today strong winds sweep across the Sahara and lift clouds of dust into the air. NASA satellites have tracked that dust traveling 1,600 miles across the Atlantic Ocean, and some of it settles over the Amazon rainforest in South America.
A desert lake filling with water is rare, but it does happen. In September 2024, a big storm dropped heavy rain on parts of northern Africa. NASA's Landsat 9 satellite watched Sebkha el Melah, a usually dry lake in Algeria, partly fill with water. By October 16 it was roughly one-third full.
Using CALIPSO satellite lidar data from 2007 through 2013, NASA researchers found that winds carry on average 182 million tons of dust a year past the Sahara's western edge at longitude 15°W, and that 27.7 million tons settle over the Amazon basin.
The desert can still hold water for a while. After an extratropical cyclone in September 2024, Sebkha el Melah in Algeria covered 191 square kilometers to a depth of 2.2 meters by October 16, roughly one-third full, according to Moshe Armon of the Hebrew University of Jerusalem. Since June 2000, only two other rain events, in 2008 and 2014, produced larger lake volumes there. After the 2008 filling, the lake took until 2012 to dry completely.
Imagine a green Sahara
Let's pretend the Sahara turns green again.
Grass might grow where sand is now. Lakes might fill. Less dust might blow away. Let's find out why.
Now imagine rain returning to the Sahara, year after year. Grass and shrubs might spread across land that is sand today, and lakes like Sebkha el Melah might stay full instead of drying out.
With plants covering the ground, much less dust might blow away. During the green period long ago, the Sahara gave off about five times less dust than it does today. That might change what reaches the Amazon, because Saharan dust carries a plant nutrient called phosphorus. These are ideas: we reason from what scientists know about the past.
Suppose monsoon rainfall over North Africa strengthened enough to bring back savanna-type vegetation. Lakes might persist, and dust emissions might fall: sediment records off northwest Africa show the Sahara emitted about five times less dust during the African Humid Period than today (McGee and colleagues, MIT News).
A possible knock-on effect concerns the Amazon. NASA estimates that dust from the Sahara delivers about 22,000 tons of phosphorus a year to Amazon soils, about the same amount the basin loses to rain and flooding. A greener, less dusty Sahara might therefore deliver less of that nutrient. These are reasoned ideas, not predictions; their size and timing are unknown.
Why the Sahara was green
Earth wobbles very slowly on its axis.
The wobble changes how much sunshine Earth gets in summer. Long ago, it brought more summer rain to the Sahara.
Earth wobbles very slowly on its axis. This wobble is called precession, and it repeats about every 21,000 years. It changes how much energy Earth receives in each season.
Researchers at the Universities of Bristol and Helsinki found that the wobble brought warmer summers to the Northern Hemisphere. That strengthened the West African Monsoon, a season of heavy rain, so more rain fell on the Sahara and savanna plants spread across it. Their model showed humid periods every 21,000 years over the past 800,000 years, but not during the ice ages, when huge ice sheets cooled the air.
Paleoclimatologists generally accept that small orbital variations, the Milankovitch cycles, were key drivers of the African Humid Period, shifting the distribution of sunlight and the strength and position of the North African monsoon (NASA Earth Observatory).
Armstrong and colleagues (Nature Communications, 2023; University of Bristol and University of Helsinki) simulated North African humid periods over the past 800,000 years. Precession, the way Earth wobbles on its axis over a roughly 21,000-year cycle, set the timing: warmer Northern Hemisphere summers intensified the West African Monsoon and spread savanna-type vegetation. Large high-latitude ice sheets suppressed the humid periods during glacial times. Many models struggle to produce enough rain to fill as many lakes as geologists think existed, so just how wet the Sahara became is still debated.
Reading the clues
Scientists study mud from the seafloor to learn about old dust.
They also watch desert lakes from space. Each lake is a clue about rain.
How do we know the Sahara was green? Scientists pulled up long tubes of mud from the seafloor off northwest Africa. Some of that mud is dust blown from the land, layer by layer. In these cores, about one centimeter of mud stands for about 100 years.
Scientists also use satellites to watch desert lakes fill and dry. Moshe Armon, a scientist who studies them, explains that these lakes work a bit like giant rain gauges, giving clues about how much rain fell long ago. Whether the Sahara might green again in the future is still very unclear.
Evidence comes from several archives. David McGee's team used thorium-230 normalization and grain-size modeling on ocean sediment cores to measure past dust; about a centimeter of sediment represents about 100 years. They found the humid period began and ended very abruptly, nearly simultaneously across North Africa, consistent with the earlier view that the region dried within one to two centuries (MIT News).
Today, satellite records of lakes such as Sebkha el Melah help test ideas about past rainfall. Armon has proposed that extreme rain events may have been more frequent in the past. For the future, he notes that IPCC projections show some parts of the Sahara wetter and others drier, with uncertainties larger than the projected changes.
Amazing facts
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Sahara dust could fill more than 100,000 big trucks every year!
The 27.7 million tons of Saharan dust that fall on the Amazon each year could fill 104,908 semi trucks.
NASA likens the 27.7 million tons of dust settling over the Amazon each year to 104,908 semi trucks, and the 182 million tons leaving the Sahara to 689,290.
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Mud from the seafloor keeps a record of dust, layer by layer.
In seafloor mud off northwest Africa, about one centimeter of mud stands for about 100 years.
In the cores McGee analyzed, a 10-foot column spans about 30,000 years, so a centimeter represents about 100 years of deposition.
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After a big rain in 2008, a Sahara lake took years to dry up.
After Sebkha el Melah filled in 2008, it took until 2012 to dry out completely.
Once filled, Sebkha el Melah holds water for a long time: the 2008 filling did not dry completely until 2012 (NASA Earth Observatory).
Words to know
- desert
- a very dry place where little rain falls
- monsoon
- a seasonal shift of winds that brings a rainy season
- African Humid Period
- a time, roughly 11,000 to 5,000 years ago, when the Sahara was much wetter and greener
- precession
- the slow wobble of Earth on its axis, which repeats about every 21,000 years
- sediment core
- a long tube of mud or rock pulled up in layers to study the past
- phosphorus
- a nutrient that plants need to grow
Discovery challenge
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When was the Sahara green?
When was the African Humid Period, the Sahara's green time?
NASA and MIT date the African Humid Period to…
- About 11,000 to 5,000 years ago
- About 100 years ago
- About 65 million years ago
Show the answer
About 11,000 to 5,000 years ago
Thousands of years ago, from about 11,000 to 5,000 years ago.
It likely lasted from about 11,000 to 5,000 years ago.
…roughly 11,000 to 5,000 years ago (NASA Earth Observatory; MIT News).
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Which animal do old Sahara cave paintings show in watering holes?
Which animals do ancient Sahara cave paintings show in watering holes?
Rock art from the green Sahara depicts which animals in watering holes?
- Hippos
- Penguins
- Polar bears
Show the answer
Hippos
Hippos! The paintings show elephants and giraffes too.
Hippos, along with herds of elephants and giraffes.
Hippos, with roving herds of elephants and giraffes (MIT News).
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What slow movement of Earth helped bring rain to the Sahara?
What slow change in Earth's motion set the timing of the green Sahara periods?
Which orbital factor paced the North African humid periods in the 2023 model?
- A slow wobble of Earth on its axis
- Earth stopping its spin
- The Moon moving closer
Show the answer
A slow wobble of Earth on its axis
Earth's very slow wobble on its axis.
Precession, a wobble of Earth on its axis that repeats about every 21,000 years.
Precession, on a roughly 21,000-year cycle, strengthening the West African Monsoon (University of Bristol).
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How much dust did the green Sahara make, compared with today?
Compared with today, how much dust did the Sahara give off during its green period?
Ocean sediment records show African Humid Period dust emissions were…
- About five times less
- About the same
- About ten times more
Show the answer
About five times less
Much less: about five times less dust.
About five times less dust than today.
…about one-fifth of today's (McGee and colleagues, MIT News).
Experiments to try
- Watch What If? Lab What if a town used up its groundwater? Below the ground, water fills the tiny gaps between grains of rock and sand. In the United States, this groundwater is the drinking water for about half of all people. Rain and snow refill it, but often slowly. Imagine a town pumping faster than nature refills it, and follow what might change and how people help.
- Watch What If? Lab What if clouds were made of candyfloss? Puffy clouds look like giant balls of candyfloss (cotton candy), but they are really made of tiny water droplets or ice crystals, and the water in a big one weighs about as much as 100 big African elephants. Imagine clouds spun from sugar instead, and find out what real clouds are made of.
- Watch What If? Lab What if an ice age came back? About 20,000 years ago, ice sheets more than 3,000 meters thick covered much of North America, and the sea was about 125 meters lower than today. Ice ages come and go over tens of thousands of years as Earth's orbit and tilt slowly change. Imagine an ice age returning, and follow what might change.
- Watch What If? Lab What if forests became grasslands? Forests and grasslands are next-door neighbors in many places, and fire, grazing animals and rain decide where one ends and the other begins. Imagine a forest becoming a grassland: who would move in, what would happen to the soil, and where would all the carbon in the trees go?
Explore next
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Watch
Animal
Common hippopotamus
The common hippopotamus is one of the biggest animals on land, yet it spends most of the day resting in rivers and lakes to keep cool. At night it walks out to graze, and its special skin makes its own sunscreen.
- Watch What If? Lab What if a town used up its groundwater? Below the ground, water fills the tiny gaps between grains of rock and sand. In the United States, this groundwater is the drinking water for about half of all people. Rain and snow refill it, but often slowly. Imagine a town pumping faster than nature refills it, and follow what might change and how people help.
- Watch What If? Lab What if clouds were made of candyfloss? Puffy clouds look like giant balls of candyfloss (cotton candy), but they are really made of tiny water droplets or ice crystals, and the water in a big one weighs about as much as 100 big African elephants. Imagine clouds spun from sugar instead, and find out what real clouds are made of.
- Watch What If? Lab What if an ice age came back? About 20,000 years ago, ice sheets more than 3,000 meters thick covered much of North America, and the sea was about 125 meters lower than today. Ice ages come and go over tens of thousands of years as Earth's orbit and tilt slowly change. Imagine an ice age returning, and follow what might change.
- Watch What If? Lab What if forests became grasslands? Forests and grasslands are next-door neighbors in many places, and fire, grazing animals and rain decide where one ends and the other begins. Imagine a forest becoming a grassland: who would move in, what would happen to the soil, and where would all the carbon in the trees go?
- Watch Try it What If? Lab What if Earth had no atmosphere? Earth's air works like a jacket: it keeps heat in and carries it around. Take the air away in our model of a sandy patch of ground on the equator, and the coldest part of the night drops from about 13 °C to about 2 °C.
- Watch Try it What If? Lab What if there were no greenhouse gases? Greenhouse gases such as carbon dioxide and water vapor hold some of Earth's heat in, like a blanket. Take them all away in our energy-balance model and Earth's average temperature falls from about 15 °C to about −19 °C, well below freezing.
- Watch Try it What If? Lab What if there were more greenhouse gases? Adding greenhouse gases makes it harder for Earth's heat to escape, so the planet warms until it balances again. In our model a small change in how freely heat escapes warms Earth by about 2.6 °C, and the oceans make the warming take years.
For parents & teachers
What children will learn
- The Sahara was much wetter and greener during the African Humid Period, roughly 11,000 to 5,000 years ago.
- A slow wobble of Earth on its axis (precession) strengthened the West African Monsoon and paced green periods.
- Scientists read past climate from ocean sediment cores, rock art and desert lakes.
- Thought experiments separate evidence about the past from ideas about the future.
Recommended ages: 9 to 17 · About 15 minutes
Talk about it
- How can a painting on a cave wall tell us about the climate thousands of years ago?
- Why might dust from a desert matter to a rainforest on another continent?
- Why do scientists say they are unsure whether the Sahara will get wetter or drier in future?
Try at home or in class
- Open NASA Earth Observatory's 'Water for a Desert Lake in Algeria' and compare its two Landsat pictures of Sebkha el Melah (August 12 and September 29, 2024): where has the water spread?
- Make a paper timeline strip, one centimeter for every 100 years like MIT's seafloor mud, and mark 11,000 and 5,000 years ago. How long is the green Sahara on your strip?
Key vocabulary: desert, monsoon, African Humid Period, precession, sediment core, phosphorus
Where these facts come from
- Water for a Desert Lake in Algeria , NASA Earth Observatory
- A 'green' Sahara was far less dusty than today , MIT News, Massachusetts Institute of Technology
- New research reveals why and when the Sahara Desert was green , University of Bristol
- NASA satellite reveals how much Saharan dust feeds Amazon's plants , NASA
Checked, not yet human-reviewed. Last updated October 24, 2026. Spotted a mistake? Tell us.